where START is starting positing in seconds or in format hh:mm:ss
LENGTH is the chunk length in seconds or in format hh:mm:ss
So you will need to run this command few times depending on how long
your video. If let's say your video is 31 minutes long and you want so
split into 15 min chunks here is how you run it:

There is a python script that you can use that does this
automatically(i.e. takes video file, chunk size in seconds and generates
individual playable video files): http://icephoenix.us/notes-for-myself/auto-splitting-video-file-in-equal-chunks-with-ffmpeg-and-python/